This invention relates to general service incandescent lighting units, and more particularly, to a higher efficiency general service incandescent lighting unit having an improved capacitive ballast circuit.
The continuing pursuit of improving the efficiency of lamps is of increasing importance due to the increasing cost of energy. One of the family of lamps in which efficiency is desired to be improved is the incandescent lamp. Incandescent lamps, although having efficiency ratings lower than those of fluorescent and high intensity discharge lamps, have many attractive features, such as, low cost, compact size, instant light, dimmability, convenience, pleasing spectral distribution, and millions of existing sockets in the homes of the users who have become accustomed to the pleasing incandescent type lighting.
The efficiencies of the incandescent lamp have recently been improved and such improvements are described in U.S. patent application Ser. No. 519,162 and U.S. Pat. Nos. 4,517,491; 4,535,269; and 4,524, 302 all filed Aug. 1, 1983 and assigned to the same assignee as the present invention.
The hereinabove referenced U.S. patent application Ser. No. 519,165 discloses a filament operated at a low voltage while maintaining the wattage and even increasing the efficacy of the general service incandescent lamp containing the filament.
The operation of a filament at a low voltage may be accomplished by the use of a capacitive ballast circuit. One such ballast circuit is described in my U.S. patent application Ser. No. 379,411, now U.S. Pat. No. 4,516,056 filed May 18, 1982 and assigned to the same assignee as the present invention. In such a ballast circuit a first and a second capacitive element reduce the voltage applied across a low voltage filament.
I have determined that in the use of capacitive elements to reduce the voltage across the filament, certain disadvantages can exist in the event the capacitive elements should be inadvertently left in their charged state. The disadvantages resulting from inadvertently charged capacitive elements are (1) if the plug of the lamp fixture, housing the lamp, is suddenly removed from the wall receptacle, the potential of the charged capacitive elements may be present at the prongs of the plug, and (2) if the lamp is quickly removed from the socket of the fixture, the potential of the charged capacitive elements may be present at the base of the lamp. Additionally, if the power applied to the lamp is suddenly interrupted and then quickly reapplied, the voltage potential across the capacitive elements may be of a value, such as 440 volts, which if allowed to discharge across the filament may damage the filament.
Various means such as a resistor placed in a parallel arrangement with the capacitive elements such as to bleed off the charge stored in the capacitive elements can be a solution to the disadvantage of the capacitive ballast circuits. However, in order for the bleed-off time to be less than, for example two seconds, a resistor in the order of 20K ohms is typically required. Such a resistor is disadvantageous in that it dissipates about 0.5 watts during a steady-state operation which degrades the efficacy of the improved general service lamp. It is desired to operate a capacitor ballast circuit having means to prevent inadvertently charged capacitive elements without the need of providing devices which degrade the efficacy of the improved general service incandescent lamp.
Accordingly, an object of the present invention is to provide an improved general service incandescent lamp having a capacitor ballast circuit for a low voltage filament and having means to reduce substantially or even to eliminate the efficacy degradation of the capacitive ballast circuits.
This and other objects of the present invention will become apparent upon consideration of the following description of the invention.